Spectral emissivity normal to the sample surface was investigated as a potential intelligent control parameter for the manufacture of SiC fiber reinforced SiC composites by chemical vapor infiltration (CVI) and microwave assisted CVI. Results indicated that at temperatures between 600 and 1000°C and wavelengths between 1500 and 2100 nm emissivity was sensitive to the sample porosity. It also appeared to be sensitive to the thickness of the oxide layer on the composites. The emissivity was not very sensitive to temperature in this region. It was concluded that emissivity is promising as a probe of density during manufacturing. It may also be useful as a probe of oxide layer formation.

Nicalon fiber is the primary reinforcement in SiC-SiC composites currently produced by a variety of techniques including CVI and polymer infiltration. Low strength retention and dimensional change at high temperatures of the Nicalon fibers limits the choice of manufacturing processes which can be employed to produce low cost SiC-SiC composites. MER has developed a SiC reinforcement based upon the conversion of low cost carbon fabric to SiC via a Chemical Vapor Reaction (CVR) process. These new SiC filaments exhibit excellent creep resistance at temperatures up to 1600°C. SiC-SiC composites were fabricated using different types of graphite fabric converted to SiC fabric utilizing the CVR process combined with a polycarbosilane (PCS) infiltration and CVI densification. In addition, enhancement of the composite through-the-thickness thermal conductivity was accomplished via boron doping of the matrix. A correlation between processing conditions, microstructure and properties of the SiC-SiC composites will be presented.

In this paper we demonstrate a liquid exchange process to introduce a ductile metal reinforcement phase in the amount of 10–30 vol. % into reaction-bonded silicon carbides (RBSCs). Immersion of RBSC in pure Al or Al-Si melts enables diffusional replacement of secondary phase silicon with metal. The Al and Al-Si exchanged composites show improvement in fracture toughness (single edge precracked beam technique) to 6–7 MPa·m1/2 as compared to 3–4 MPa·m1/2 in otherwise similar siliconized silicon carbide. Increased fracture strength (four point flexure) was also observed after the liquid exchange process.

The pyrolysis of a poly ethylsilazane was studied using a CO2-laser beam heated solid state MAS-NMR probe head. Chemical structures of the intermediate stages could be identified. The analogy of the pyrolysis evolution for the following two different methods could be shown: A) in an inert gas furnace conventionally prepared and B) laser irradiated in the NMR probe head under magic-angle spinning (MAS) conditions. Samples prepared by method A were studied by 29Si MAS-NMR and samples prepared by method B were studied with an appropriate cross polarization time by 29Si CP-MAS-NMR. Both experiments showed the same mechanisms for the pyrolysis as the polymer is transformed into a Si3N4/C ceramic.

The reactions taking place between coarse grained mixtures of silica (cristobalite) and carbon (graphite) at 1558°C in pure CO as well as mixtures of CO and Ar, have been subjected to a thermogravimetric investigation. A model is developed in the regime where the formation of SiC does not take place. The primary steps are assumed to be: Reaction betweefn SiO2 and CO to give SiO(g) and CO2 followed by the reaction between CO2 and C to give CO. The model predicts the prevailing partial pressure of SiO throughout the charge, and the correlation between observations and model strongly supports the above given reaction mechanism.

Enhanced and accelerating reaction rates were observed when the formation of β-SiC took place. It is suggested that this is due to the continuous formation of stable SiC-nucleus on the Csurface and the subsequent shortening of the diffusion path for SiO.

Chemical vapor infiltration is a unique method for preparing continuous fiber ceramic composites that spares the strong but relatively fragile fibers from damaging thermal, mechanical, and chemical degradation. The process is relatively complex and modeling requires detailed phenomenological knowledge of the chemical kinetics and mass and heat transport. An overview of some of the current understanding and modeling of CVI and examples of efforts to optimize the processes is given. Finally, recent efforts to scale-up the process to produce tubular forms are described.

Since NH3 is an important component of TiN chemical vapor deposition (CVD) processes, understanding the NH3/TiN surface interaction is crucial to developing a model for the overall reaction. Temperature programmed desorption experiments show that NH3 adsorbs molecularly on amorphous TiN surfaces. Chemisorption occurs at only ∼5% of the surface sites, with an activation energy for desorption of 24 kcal/mol. The sticking probability into this state is 0.06 at 100 K. In addition, NH3 adsorbs with high probability into a multilayer state with an activation energy for desorption of 7.3 kcal/mol, similar to that found in other systems. NH3 does not dissociate on TiN. Under CVD conditions, however, the reactivity of NH3 on TiN may increase and surface reactions may play a part in film growth.

The kinetics of gas-phase reactions occurring during the CVD of boron nitride (BN) from BCl3 and NH3 are investigated using an elementary reaction mechanism whose rate constants were obtained from theoretical predictions and literature sources. Plug-flow calculations using this mechanism predict that unimolecular decomposition of BCl3 is not significant under typical CVD conditions, but that some NH3 decomposition may occur, especially for deposition occurring at atmospheric pressure. Reaction of BCl3 with NH3 is rapid under CVD conditions and yields species containing both boron and nitrogen. One of these compounds, Cl2BNH2, is predicted to be a key gas-phase precursor to BN.

Measurements of the decomposition of mixtures of boron trichloride (BCl3), titanium tetrachloride (TiCl4), and hydrogen at elevated temperatures are presented. The decomposition of BCl3 with hydrogen appears to drive the chemistry in this system. The species depositing boron on the surface contains at least 2 chlorine atoms. Once deposited, the surface chlorine is removed by reaction with hydrogen to form HCl and, presumably, surface B-H bonds.

Chlorine-containing precursors are attractive for chemical vapor deposition (CVD) of SiC because they are less hazardous and more economical than silane precursors. The reactivity of HCl, a by-product of these reactions, on SiC is of particular interest because it has been reported that HCl inhibits SiC CVD, but the mechanism for this inhibition has not been identified. In this work the adsorption of HCl on polycrystalline β-SiC was examined with Auger Electron Spectroscopy (AES) and Temperature Programmed Desorption (TPD). HCl adsorbs readily on SiC, with an initial sticking probability of 0.1 at 300 K, and forms a strong bond, with an activation energy for desorption of 64 kcal/mol. The only product detected by TPD is HCl, which desorbs in a peak centered at 1010 K. There are no Si- or C-containing desorption products, demonstrating that HCl does not etch SiC under TPD conditions. These results are consistent with a site-blocking mechanism for HCl inhibition of SiC CVD, but not with an etching mechanism.